The role of the complement system in immune responses-infectious challenge, cell and humoral mediated reactivity - is being assessed using genetically controlled variants as identified in different mouse strains. C5-deficient and a congenic C5-sufficient line and a closely related high hemolytic complement line (designated "Supermouse") provide the experimental materials. Additionally, the complement-inactivating properties of cells derived from different tissues are being studied as a clue to what may be an important regulatory mechanism modulating the role of complement in vivo.